There is a long felt and continuing need in the aircraft industry to control and reduce interior noise levels in aircraft and other vehicles. This need is constrained by restrictions in weight, in volume and in cost. Nevertheless, lack of effective noise control results in operator and passenger fatigue and discomfort. Often, noise also creates a hazard, either by distraction or by direct interference, for example interference with intra-cockpit or radio communications. Additionally, high noise levels excite vibrational modes in aircraft structures and skin materials resulting in mechanical wear and stress. Because of the wear and stress, heavier structures are required with the accompanying penalties of larger engines, more fuel, etc.
Solutions to aircraft noise problems have been difficult partially because of the complex interaction of noise frequencies traveling through the interior cabin air and through the structures of aircraft. Control of noise being transmitted through the interior cabin airspace is basically a function of (1) isolation breaking the acoustic path, (2) insulation with mass for low frequency noise and (3) damping with multi layers of soft material for higher frequency noise. Control of noise being transmitted by the vehicle structure is basically a function of (1) providing discontinuities in the hard structures and (2) introducing resilient joints between structural components. Control of both types of transmission is also affected by the acoustic impedance of boundary materials, that is, the ratio of sound pressure to the corresponding particle velocity at the boundary surface. The above factors interact to determine whether noise will be transmitted from the outside air to the aircraft skin and structural components, where noise will be conducted within the structures, and whether it will be re-emitted into the interior air of the aircraft.
Compounding the intractability of the noise problem has been a lack of adequate acoustic test devices. Prior art methods have included two basic methods of testing. In the first method specific aircraft parts or materials are tested in various small acoustic chambers. A piece of carpeting, panel or door, for example can be tested in order to compare one sample against another. Unfortunately, this method frequently interferes with the normal vibrational dynamics of the installed sample. Further, this method fails to account for sound path conduction and re-emission. For example, an aircraft door may pick up external noise and transmit it structurally through hinges and latching mechanisms to other parts of the aircraft where the noise is re-emitted into the cabin space. This phenomenon may occur despite heavy acoustic insulation within the door which prevents direct noise transmission into the cabin.
In the second general method of testing, an assembled aircraft is instrumented and tested as a unit. This testing can be accomplished either by flight or wind tunnel testing or by using large static acoustic chambers. By this method the difficulties of changing vibrational dynamics and of severing noise conduction channels are avoided. However, the inability to restrict noise impingement to selected portions of the aircraft prevents accurate determination of the noise source. Gross noise levels can be determined, but the source cannot. For example, in a large test chamber, noise impinging the entire aircraft may be picked up by a tail surface, transmitted by longeron to the cabin area, and emitted into the cabin air space despite the presence of multiple layers of acoustic insulation within the cabin walls.
Accordingly, it is an object of the present invention to provide a method which will allow testing of an assembled vehicle while retaining the ability to selectively exclude parts of the vehicle from direct sound impingement.
Another object of the present invention is to provide a device to accomplish selective, full-scale acoustic testing.
It is a further object of the present invention to provide a method to accomplish selective testing of a aircraft or vehicular component, such as a door or window, without removing the component from the vehicle and without disrupting conductive sound paths.
Yet another object of the present invention is to provide a device, containing a noise source, an acoustic guide to selectively direct noise impingement, and measurement microplanes, which will allow acoustic testing and analysis of vehicular components.